jewel vs crown jewel

jewel and crown jewel both are nouns.

Word	Noun	Adjective	Verb	Adverb
jewel	Yes	No	Yes	No
crown jewel	Yes	No	No	No

As nouns, crown jewel is a hyponym of jewel; that is, crown jewel is a word with a more specific, narrower meaning than jewel:

jewel: a precious or semiprecious stone incorporated into a piece of jewelry
crown jewel: a precious stone that is a valuable part of a sovereign's regalia

Other hyponyms of jewel include solitaire, diamond, ruby, pearl, emerald, sapphire.

jewel (noun)	crown jewel (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a precious stone that is a valuable part of a sovereign's regalia
a person who is as brilliant and precious as a piece of jewelry	the most desirable assets of a corporation

jewel (verb)	crown jewel (verb)
adorn or decorate with precious stones